It has been suggested to provide between the two ducts in a spray booth a heat exchanger for improvement of the operation economy, especially during the winter season. However, experiments in this direction have not arrived at practical results i.e. due to the fact that the exhausted moist air by becoming cooled down in the exchanger causes formation of ice in the passageways of the exchanger so as to clog them. A spray booth is normally composed of a plurality of compartments or units each of which has a spray gun and inlet and outlet ducts for air and, in addition, means for generation of a water curtain. The booth or the units thereof are in operation for spraying during a minor portion only of a working-day. During the remainder of the working time these members thus are idle-running, i.e. the water curtain is kept operative which results in that the exhaust air continuously takes up moisture while its temperature is reduced. When this air during the cold season passes through the heat exchanger, it becomes cooled further so that its temperature falls below the dew or freezing point resulting in that ice is precipitated and clogs the passageways of the heat exchanger. The entering admitted air must be supplied with heat from a separate heat source both ahead of and behind the heat exchanger to eliminate the danger of ice-formation in the heat exchanger and to keep the interior of the booth at an acceptable temperature.